Nonwoven fabrics which contain fibers having different melting temperatures are well known in the field of the nonwoven fabrics. The fibers with the lower melting point act as an adhesive agent which bonds the higher melting-point fibers to each other. Fibers containing polyethylene and polypropylene are often used to manufacture nonwoven fabrics because of their desirable characteristics such as comparatively low melting point, a strong bond between fibers and a good hand of the fibers. However, since polyethylene is difficult to be spun into filaments at high speed, it has not been easy to produce polyethylene/polypropylene containing nonwovens in accordance with a spunbonding process comprising continuous spinning and web forming operations. Low-density and high-density polyethylenes have been used as polyethylene fibers. It has recently been proposed that a linear low-density polyethylene (hereinafter abbreviated as LLDPE) prepared by copolymerizing ethylene and an .alpha.-olefin having 3 to 12 carbon atoms as described in Japanese Patent Application (OPI) No. 209010/85 (the term "OPI" as used herein means an "unexamined published Japanese patent application") or a blend of specified proportions of low-density polyethylene and crystalline polypropylene (as described in U.S. Pat. No. 4,632,861) are used in the production of polyolefin fibers. A growing need for spinning at higher speeds exists today not only in the art of making spunbonded nonwoven fabrics but also for the purpose of reducing the production cost of multi-filaments. However, the LLDPE shown in Japanese Patent Application (OPI) No. 209010/85 which is specified to have a density and a melt index (hereinafter referred to as MI value) within certain ranges is difficult to spin at high speed and its spinnability is also unsatisfactory. Fibers of fine denier can be spun at high speed by employing a spinning temperature that is much higher than the melting point of LLDPE, but at the same time, the surface of spinneret tends to be soiled over time, thereby inducing such troubles as kneeling and filament breakage.
As regards the blend of low-density polyethylene and polypropylene, U.S. Pat. No. 4,632,861 discloses a process for producing blend fiber and nonwoven fabric by the following procedures: melt-spinning the blend to form a first group of filaments; quenching the first group of filaments; bringing the first group of filaments together with a second group of filaments which have a higher melting point than the polyethylene component of the first group of filaments; forming a composite web of the two groups of filaments; and bonding the web by compressing the web while heating it to a temperature above the melting point of the polyethylene component. Although the low-density polyethylene specified in this patent has a melting point of less than 107.degree. C., the melt spinning of the blend is performed at a temperature in the range of 205 to 265.degree. C., preferably between 230.degree. C. and 260.degree. C. This should increase the chance of the surface of spinneret of becoming soiled over time, thereby inducing such troubles as kneeling and filament breakage. Furthermore, the highest spinning speed that could be attained was only 4,600 m/min for a given blend ratio at a spinning temperature of 260.degree. C.
In the production of nonwoven fabrics, fiber-to-fiber bonds are introduced by interlacing fibers as in the case of needle punch method or by employing a variety of adhesive agents as binders. In applications such as inner cover-stock for disposable diapers and sanitary napkins the demand for which has increased rapidly these days, the nonwoven fabrics are required to satisfy various properties such as soft hand (comfortable touch to the skin), lightweightness (light weight) and high tensile strength. In order to satisfy these requirements as much as possible, the binder method has been employed as a common technique for producing nonwoven fabrics. The principal approach of binding has been to deposit a solution of adhesive agent on webs but this suffers from such disadvantages as the extra need for energy in order to remove the solvent used in the solution of adhesive agent and the fouling of a working environment due to solvent stripping.
With a view to solving these problems, it has been proposed that a first group of fibers constituting a web is bonded together by incorporating in the web a binder in the form of a second group of fibers having a lower melting point than the first group of fibers, with the web being then subjected to a heat treatment. For example, Japanese Patent Publication No. 10583/86 proposes that a bicomponent fiber made of fiber-forming polymers having different melting points is used as a binder for nonwoven fabrics having high strength and good hand. Low-density polyethylene (LDPE) and high-density polyethylene (HDPE) are commonly used as the bicomponent fiber-forming polymers but nonwoven fabrics produced using these polyethylenes are unsatisfactory in that they have rigid hand and fail to offer a soft feel. In order to eliminate this defect, it has been proposed in Japanese Patent Application (OPI) Nos. 209010/85 and 194113/85 that LLDPE fibers prepared by copolymerizing ethylene with octene-1 be used as a binder for nonwoven fabrics because they offer soft hand and have a low melting point. However, staple fibers made by using LLDPE as a binder are only insufficiently crimped to provide a desired degree of bulkiness.